This invention relates to lasers and in particular to such lasers in which the optical cavity is "folded" into a number of sections. One of the factors affecting the output power of a laser is the length of the optical path passing through an excited active medium. Hence the length of the optical path is dependent upon the required output power. The physical dimensions of the laser will be determined by other considerations, however, and it is common to "fold" the optical path of a laser to reduce its actual length. Folding is achieved by using prisms or mirrors at the end of each laser section, and it is necessary to ensure that the correct alignments of these reflectors are maintained. This is a serious problem which increases as the number of folds increases.